1. Field of the Invention
This invention relates to an indexable cutting insert which may be attached to various kinds of insert cutters.
2. Related Art
An end mill that uses a conventional, indexable cutting insert comprises a cylindrical tool body with an axis of rotation and an axially forward end portion with a reduced diameter. A tip pocket, with a diameter that increases gradually in an axially rearward direction, is formed in the forward end portion. An insert-receiving recess is formed in the foremost corner of a bottom of the tip pocket. The corner is located at an axially forward position facing in a direction of the tool body's rotation. An indexable cutting insert fits into an insert-receiving recess and is secured therein by a clamp screw.
The insert is of a positive type. It comprises a plate-like insert body with a pair of generally parallelogrammic front and back faces disposed parallel to each other. The front face serves as a rake surface. The back face serves as a seating surface held in contact with a bottom of the insert-receiving recess. The front face includes a diagonally opposite pair of acute corners formed into a pair of nose portions. The insert body also has two adjacent pairs of side faces lying between the front and back faces. Each adjacent pair of side faces sandwiches a respective nose portion therebetween and intersects the front face at a pair of marginal ridges. The side edges define a peripheral cutting edge and an end cutting edge, which extend directly from the nose portion. A central mounting aperture extends through the thickness of the insert body.
The insert-receiving recess conforms to the outer shape of tile insert body so that, when the insert is secured, the peripheral and end cutting edges and the nose portions protrude from the tool body and can be indexed to active cutting positions. The insert-receiving recess has a flat bottom surface inclined to the axis of the tool body so that the thickness of the forward end portion increases gradually in an axially rearward direction. The recess includes a pair of walls that merge into an inner corner. An internally threaded aperture in the center of the flat bottom surface shifts its axis slightly toward the inner corner from the axis of the central mounting aperture of the insert body when the insert is fitted into the recess. An elliptical recess is formed at the inner corner of the insert-receiving recess.
For mounting the insert on the tool body, the insert is first fitted into the insert-receiving recess. The clamp screw is then turned to bring the back face of the insert body snugly against the flat bottom of the recess. As a result, the shift between the axis of the internally threaded aperture of the insert-receiving recess and the axis of the central mounting aperture of the insert body presses the insert body toward the inner corner of the recess, with the side faces that sandwich the inactive nose portion being held against the walls of the recess. The peripheral and end cutting edges protrude from the forward end portion of the tool body and are indexed properly into peripheral and end cutting positions respectively.
In such an end mill, the insert bears a great cutting load when it engages a work piece. Hence the forward end portion of the tool body, on which the insert is mounted, must be designed to have sufficient rigidity. For this reason, the tip pocket is formed, and the flat bottom of the insert-receiving recess is inclined to the axis of the tool body, so that tile thickness of the forward end portion increases gradually in an axially rearward direction.
However, when the insert, with its parallel rake and seating surfaces, is secured to the insert-receiving recess with its flat bottom inclined as described, the rake surface becomes negatively inclined to the axis of the tool body. Deterioration of cutting performance is the result.
The radial rake angle of the cutting insert is determined by the thickness of the forward end portion of the tool body and the thickness of the insert, for a given outer diameter of the forward end portion of the tool body, on which the insert is mounted. If the forward end portion is to have sufficient rigidity, its thickness cannot be reduced too much. Nor can the thickness of the insert be substantially reduced, since it, too, must be rigid. Thus tile radial rake angle can become unduly negative and cutting performance thereby further deteriorated in an end mill of conventional design.